The present invention relates to a method of manufacturing a heat exchanging fin having an oval collared through hole, in which an oval heat exchanging tube is inserted, and a die set for manufacturing said heat exchanging fin.
In a heat exchanger for a room air conditioner or a car air conditioner, a plurality of heat exchanging fins, which are made of, for example, thin aluminum plates, are piled. Each of the heat exchanging fins has a plurality of collared through-holes. Heat exchanging tubes are inserted in the collared through-holes. Generally, the heat exchanging tube is made of a high heat conductive material, e.g., copper.
Conventionally, tubes having oval transverse sectional shapes have been used as the heat exchanging tubes. Note that, the word “oval” means not only an elliptical shape but also an egg shape, and the “oval” shape has a major axis, a minor axis and no apexes.
A merit of the oval heat exchanging tubes will be explained. If the oval heat exchanging tubes are inserted in the oval collared through-holes of the heat exchanging fins so as to make an air streaming direction parallel to the major axes of the oval heat exchanging tubes, air can smoothly stream between the heat exchanging fins so that heat exchanging efficiency can be improved.
Conventionally, the heat exchanging fin having the oval collared through-holes is manufactured by drawing a thin metal plate. The method will be explained with reference to FIGS. 7A–7F.
In the step shown in FIG. 7A, an oval projected section 13, whose sectional shape is a trapezoidal shape of a square shape, is formed in the thin metal plate 10, e.g., a thin aluminum plate, by an oval drawing die and an oval drawing punch (not shown). A major axis and a minor axis of the oval projected section 13 are greater than those of a desired oval hole 12 (see FIG. 7F). To form the oval projected section 13, the oval drawing die and the oval drawing punch have oval transverse sectional shapes.
The oval projected section 13 formed in the step shown in FIG. 7A is further drawn in the steps shown in FIGS. 7B–7D so as to reduce the major axis and the minor axis. By the drawing steps, the oval projected section 13 is formed into an oval projected section 14 having a prescribed height, a prescribed major axis and a prescribed minor axis.
In the step shown in FIG. 7E, a through-hole is bored in an upper face of the oval projected section 14 by a pierce die and a pierce punch (not shown). By forming the through-hole, a collar section 15 is formed.
In the step shown in FIG. 7F, a front end of the collar section 15 is outwardly bent to form a flange section 16.
By above described steps, an oval collared through-hole 18 is formed in the thin plate 10.
In the conventional method, the oval projected section 13 is initially formed, then the oval projected section 13 is drawn several times (see FIGS. 7B–7D) to form the oval collared through-hole 18 having the prescribed size.
However, the oval drawing die and the oval drawing punch have complex shapes, so they must be more expensive than a circular drawing die and a circular drawing punch. Therefore, manufacturing cost must be higher; reduction of the cost of manufacturing the heat exchanging fin having the oval collared through-holes is required.
Further, in the conventional method, the oval projected section is drawn in each step shown in FIGS. 7B–7D, so that the size of the oval shape can be smaller. The oval shapes drawn in the steps shown in FIGS. 7B–7D are similar to the oval shape of the projected section 13 formed in the step shown in FIG. 7A. However, curvature of the oval shape is partially different, so thickness of the projected section is partially different, the projected section is apt to be cracked, and creases are formed in the thin plate.
A cause of those disadvantages will be explained with reference to FIG. 8.
In the oval projected section, curvature at an end 11 of the minor axis of the oval shape is different from curvature at en end 19 of the major axis thereof. If the oval projected is uniformly drawn with reduction width “d”, drawing rate at the end 11 is different from that at the end 19. For example, the drawing rate “m (%)” at the end 19 of the major axis is m=100(h2/h1). On the other hand, the drawing rate “n (%)” at the end 11 of the minor axis is n=100(i2/i1). If the reduction width “d” is equal at the ends 11 and 19 (h1−h2=i1−i2=d), the drawing rate “m” at the end 19, at which the curvature of the oval shape is smaller, is less than the drawing rate “n” at the end 11, at which the curvature of the oval shape is greater. Therefore, even if the drawing can be executed at the end 11, the oval projected section is apt to be broken at the end 19.